The invention relates to a pressure-regulating valve for regulating the pressure in a high-pressure fuel accumulator of an internal combustion engine, in particular in common-rail injection systems. A pressure-regulating valve of this type has a valve piston which is guided in an axially displaceable manner in a bore and acts on a valve-sealing element such that the valve-sealing element is pressed against a valve seat or can be raised from the valve seat in order to open the pressure-regulating valve. The adjusting pressure here is a function of the valve stroke. The axial displacement of the valve piston within the bore is brought about by a magnet coil which, upon being energized, moves the valve piston in the direction of the valve seat, i.e. in the closing direction. In the process, the compressive force of a spring which acts upon the valve piston in the opening direction is overcome.
Pressure-regulating valves for regulating the pressure in a high-pressure fuel accumulator and which are used in particular in common-rail injection systems have to meet certain safety requirements in order to protect people and the environment. For example, it has to be ensured that, in the event of errors in the control device and/or voltage supply, the pressure is limited to a permissible maximum limit value. Furthermore, in the event of a loss of voltage, for example if a cable breaks or due to damage by a marten, it should be ensured that rail pressure continues to be built up in order to permit an emergency running mode (what is referred to as the limp home function).
In the case of “currentlessly open” valves which are frequently used in common-rail injection systems of passenger vehicles, a pressure-limiting function can be realized in a simple manner by the valve opening when there is a loss of voltage. However, the open position of the valve when there is a loss of voltage has the consequence that it is generally not possible to build up a sufficient rail pressure in order to permit an emergency running mode.
The direction of action is the other way around in “currentlessly closed” valves. That is to say, the valve is closed when there is a loss of voltage. The closing of the valve is brought about by a powerful spring which acts upon the valve piston in the closing direction. The spring force of the spring is configured in such a manner that it permits adjustment of the valve piston counter to the pressure in the high-pressure fuel accumulator. In order to open the valve, a magnet coil, the magnetic force of which opposes the closing force of the spring, is energized. When there is a loss of voltage, a closed position of the valve and therefore an emergency running mode are therefore ensured. However, due to the failure of the regulating function, the pressure in the high-pressure fuel accumulator may rise in such a manner that the permissible system pressure is exceeded. Furthermore, the temperatures in the fuel return of the pressure-regulating valve may rise to values above 200° C. In the process, not only can the system components be damaged but people and the environment may also be put at risk. Currentlessly closed pressure-regulating valves, as are frequently used in common-rail injection systems of commercial vehicles, therefore regularly require the use of an additional pressure-limiting valve in order to meet the safety requirements.
Laid-open specification DE 103 34 615 A1 discloses a currentlessly open pressure-regulating valve of the type in question which, if an electromagnetic closing force fails, is intended to permit a minimum pressure at the valve outlet and therefore an emergency running mode. For this purpose, the opening cross section of the inlet of the pressure-regulating valve is configured in such a manner that the pressure-regulating valve opens only from a predetermined minimum fuel pressure. As an alternative, a throttle element is proposed in the region of the inlet in order to regulate a minimum pressure from which the pressure-regulating valve opens.